Formation of Intermetallic Compounds in a Cold-Sprayed Aluminum Coating on Magnesium Alloy Substrate after Friction Stir-Spot-Processing

Magnesium-based alloys are appropriate materials for transportation applications owing to their high strength-to-weight ratio. However, the poor corrosion performance of Mg alloy limits its lifetime. In this study, the pure Al coating was deposited onto an AZ91D substrate by cold spraying, and then modified by friction stir-spot-processing. The interfacial microstructure and phase were studied in detail, and the effect of the intermetallic compounds on the microhardness and corrosion properties of the coatings was investigated. Results show that the sound and dense coatings were obtained after friction stir-spot-processing at 2400 and 2700 rpm. Al₁₂Mg₁₇ was found in the upper part of the 1/2 radius zone in the processed coating. The thin and thick IMCs were identified at the interface in the center and 1/2 radius areas, respectively. In the thick IMCs, mainly two types of multi-phase coexistence structures were observed. The four-phase coexistence structure dominated by Al₃Mg₂ revealed a higher microhardness than the three-phase coexistence structure dominated by Al₁₂Mg₁₇. In the processed coating, the upper part of the 1/2 radius zone containing Al₁₂Mg₁₇ exhibited a higher microhardness than the center zone. Moreover, the cold-sprayed Al coating provided more effective corrosion protection for the AZ91D substrate after FSSP.